Don't take this the wrong way or anything, but it seems a little unusual to me for someone interested in "retrocomputing" to be frightened of learning gruesome technical details about how computers "in general" work. If you look at the fanbase of other old-tyme-y computers you'll find that almost every individual in those communities is "frighteningly technical" and not afraid to get their hands dirty in anything that enables them to get more out of their old machines. (Even if that means putting aside the Atari 800 or Commodore 64 long enough to hack something together to interface said machine to their Linux/DOS/Windows box to have it emulate an impossible to find peripheral, say.)
Really, this is an easy one. One could type the recipe up to do it in about twenty minutes. But if what you really want is for someone else to put together a .dmg with a drag-and-drop package that prevents you from having to type *anything* at a terminal prompt, well... that seems a little greedy. It probably wouldn't make you happy anyway because going the "terminal" route you'd still have to learn how to use those terminal-window programs. (Which are easy enough to use by 1980 Wordstar-esque context-sensitive-letter-key-menu standards, but they're not Mac-ish.)
I remember ages ago there was a clever little program someone who had a dial-up shell account with command-line Internet access could install that could "fake" a limited SLIP connection by reformatting http requests into keypunches for "lynx", thus allowing someone to run a graphical web browser. (I forget the name of it.) In theory you could set up a serial-line-appropriate email client on the OS X end (even plain-old "mail" might do) and then write a "simple" GUI shell program to run on the Mac that would act as a specialized terminal program. Said program would interpreting the menuing system of the UNIX program, screen-scrape the output, adjust its pulldown menus appropriately, and reformat the message reading/composition text into a windowed GUI editor. That might fit in 128k better then a full-fledged email program since all the message buffering and the actual network communication would stay on the UNIX end.
But, yeah, that means programming. Pretty easy programming by most standards (That terminal program I linked to above even has source you could start with for the communication portion), but programming.